An all-solid-state thin film lithium secondary battery (hereinafter referred to as a “Li-based thin film secondary battery”) is used in various devices such as a thin film solar battery, a thin film thermoelectric element, and a radio electric-charging element. About Li-based thin film secondary batteries, a demand therefor has been rapidly increasing. The Li-based thin film secondary batteries are each typically composed of a positive electrode made of a Li-containing transition metal oxide thin film containing Li and a transition metal, a solid electrolyte made of a Li-containing phosphoric-acid compound thin film, and a negative electrode made of a Li metal thin film.
For the formation of the Li-containing phosphoric-acid compound thin film, a sputtering method is preferably used in which a sputtering target (hereinafter abbreviated also to a target) made of substantially the same raw material as constitutes the film is sputtered. The sputtering method has advantages that conditions for the film formation are easily adjusted, and the film can easily be formed on a semiconductor substrate.
About this sputtering target, for example, Patent Literature 1 discloses a target obtained to have a relative density of 90% or more and shows a reduced proportion of a Li4P2O7 phase as an impurity phase by performing CIP (cold isostatic pressing) and HIP (hot isostatic pressing) successively.
Patent Literature 2 discloses a manufacturing method of pre-sintering a powder of fine Li3PO4 particles containing water to decrease the water content by percentage in the Li3PO4 particle powder, and then sintering the powder. According to this manufacturing method, a sintered body can be obtained in which the formation of defects (voids) such as pores is restrained.